Inkjet printers can be divided into two major categories, commonly referred to as continuous inkjet and drop-on-demand (DOD) ink-jet.
In DOD inkjet printers, the ink delivery apparatus for an ink-jet printhead delivers the ink to the printhead at a slight vacuum or negative pressure known as a “back pressure”. The slight negative pressure in the printhead is desired because it prevents the ink from leaking, i.e. drooling, out of closely spaced ink discharge nozzles in the printhead, by tending to draw the ink at the nozzles back into the printhead. Moreover, it forms a slightly concave ink meniscus at each nozzle which helps to keep the nozzle clean. Typically, as stated in prior art U.S. Pat. No. 5,650,811 issued Jul. 22, 1997, the slight negative pressure in the printhead may be approximately two to three inches of water below atmospheric pressure. The patent also states that the slight negative pressure can be created by positioning an ink reservoir for the printhead below the printhead. Alternatively, the slight negative pressure can be created by using a nonlinear spring to pull a membrane bladder outward at an opening in an ink reservoir above the printhead. This latter approach is described in detail in U.S. Pat. No. 4,509,062 issued Apr. 2, 1985.
Today, most DOD inkjet printheads have an “onboard” ink reservoir. In other words, the ink reservoir is fixed atop the printhead and moves with it during the printing operation. Often, as disclosed in prior art U.S. Pat. No. 5,975,689 issued Nov. 2, 1999 and U.S. Pat. No. 5,872,584 issued Feb. 16, 1999, the negative pressure regulator is coupled with an ink replenishment mechanism, both of which are located inside the ink reservoir. However, a problem that can occur with locating the negative pressure regulator and ink replenishment mechanism inside the reservoir is that it is subject to corrosion and chemical attack by the ink inside the reservoir. Also, the mechanism may contaminate the ink.